Many industries now require that workers wear safety footwear to protect their feet against injury caused, for example, by blows from above by falling objects, or crushing such as may be caused by a vehicle wheel rolling over the foot. Almost all safety footwear currently available comprise a toe protector, usually a steel box toe (sometimes called a toe box) providing a protective arch above the toes through which any impact or crushing force applied to its top surface is transmitted to the insole on which it rests, and through the insole to the outsole and the ground. It is a requirement for official certification (details of which are given below) that the box toe shall be incorporated into the footwear during construction and shall be an integral part of the footwear. Also, provision is now commonly made to protect the metatarsus, namely the five long bones of the instep extending from the toes to the remainder of the bones of the foot. Many of the prior proposals for metatarsal protection comprise a guard that is applied to the exterior of the boot or shoe, but increasingly the protector is incorporated into the footwear, it also being arranged to transmit any forces applied to it through the insole and outsole to the ground.
The Occupational Health & Safety Association (OHSA) specifies test and performance standards that have been established by American National Standards Inc. (ANSI) which safety footwear must pass if they are to be certified by them. The Canadian Standards Association (CSA) have adopted equivalent standards. The test procedure for impact force resistance involves dropping a standard weight on to the toe portion of a specimen boot (size 9D for men's footwear and 8B for women's footwear) at a point 2.5 cm (1.0 in) from the outside tip of the toe under conditions such that it exerts a nominal impact force of specified value at a specified impact velocity. Three values of force are used, namely 101.7 Joules (75 foot pounds), 67.8 Joules (50 foot pounds), and 40.7 Joules (35 foot pounds) and footwear meeting one of these standards is certified with the designation I/75, I/50 or I/35 respectively. In the crushing force test procedure the toe protector is subjected to a crushing force of 1,134 Kg (2,500 lbs; 11,121 Newtons), or 795 Kg (1,750 lbs; 7,784 Newtons), or 455 Kg (1,000 lbs; 4,448 Newtons), and depending upon the level of force that the footwear withstands it is certified with the designation C/75, C/50 or C/35 respectively. With both tests the boot is not certified if either of these forces depresses the toe portion so that the clearance inside is less than 1.27 cm (0.5 in) for a men's boot and 1.19 cm (0.468 in) for a women's boot.
The equivalent impact force test for a metatarsal protector (there is no compression test) requires the force to be applied at a point 8.9 cm (3.5 in) from the outside tip of the 9D boot toe (8.6 cm or 3.375 in for the 8B boot) and be of nominal value 101.7 Joules (75 foot pounds), or 67.8 Joules (50 foot pounds), or 40.7 Joules (30 foot pounds), the required minimum clearance remaining after the test being 2.5 cm (1.0 in) for the 9D boot and 2.4 cm (0.94 in) for the 8B boot. Footwear meeting one of these standards is certified with the designation Mt/75, Mt/50 or Mt/35 respectively.
Box toes of plastics materials are available, but they are significantly thicker than the commonly used steel box toe, so that it is more difficult to incorporate them into the boot construction without the resultant boot toe appearing much more bulky, especially in side elevation. Moreover, most safety footwear is manufactured by molding the outsole to the remainder of the boot and the thicker box toe makes it impossible to use existing molds that have employed steel box toes, so that expensive new molds are required for each style of boot. Metatarsal protectors of plastics material, e.g. high density polyethylene or polypropylene (HDPE or HDPP), ABS and various proprietary nylons, are also available and again, if the standards for impact forces are to be met, the protector is such thickness that, even if the more expensive high modulus plastics materials are used, it is difficult to incorporate in safety footwear of standard sizes. The manufacture of both box toes and metatarsal protectors from plastics material has the advantage that even if of complex shape they can be molded in a single relatively inexpensive operation. There is also the possibility that protectors that are lighter in weight can also be produced. Manufacturers of safety footwear face an inherent marketing problem that whether the footwear is supplied by the employer at cost, or with a subsidy, or whether it is purchased directly by the wearer, it is basically a product for which a relatively low purchase price is required by the customer, so that minimization of manufacturing cost is of primary importance.